Put some new horns in (one of the originals was dead) they look just like the original, but made from plastic not pot metal... Anyway after fabricating a functional grounding pin in the steering wheel hub for my aftermarket wheel, the horns worked fine - both high and low note at full blast. After driving around for a few days, I go to blow the horn and get a half assed moan out of one and the other goes full bore. So I reversed the horns and fiddled with the wires a bit ( clipping onto the horns in reverse order) and got it blowing WFO both horns. Then after some driving around, again I get the crap half honk from one and the full from the other...Sounds like crap.

Sounds like your horn button is not carrying enough current to run the horns. The easiest solution would be to just add a horn relay (search the technical library). All modern cars have horn relays, it's a lot to ask of a small switch (even the original stock one) to carry all the current necessary to run a horn.

x 2 on the horn relay. After cleaning all the connections, my horns still worked intermittently. After installing a relay in the circuit, the stock horns not only work, they are louder than I ever imagined they could be.

I've got the same plastic horns but already converted to relay operation with the old metal ones still in. Both types give one hell of a sound level with a relay nearby, pulling the earthwires to mass, + feed wires from the fusebox are thick enough already.

PS Horns have a very high starting current and exhibit an inductive load, putting a high strain on the steeringwheel switch

Peak current may well lie somewhere between 30 and 40 Amps for the double horns, but most ISO relays have 40A on NO (30-87) and 30A on NC (30-87a) contacts.
Bosch are fine, but also some others like my Goodsky and FRC. It would not surprise me if all come from China despite their German spec sheets

See Goodsky in the appended doc BTW Prices for single relays are in the 5€/$ ballpark

PS For horn and HB/LB operation normal connection is quite enough, for some other applications adding a connector aids fast exchange in case of failure
The relay preventing starting in gear (Astley's idea) and my own invented addition of a wiper switch-over relay are both on connectors. The last one saved my vacation on an unknown wiper interruption

In reply to # 3591465 by KozyB
Hi Clark, the relay I use for my horns is a Bosch-style four pin SPST 12VDC/40A. I picked it up years ago at either a Radio Shack or an automobile parts store.

Forty amps may be overkill, but the thing weighs (and costs) next to nothing.

Maybe none, constant current is not so high and activation of the horns is hopefully not so frequent. My peak current estimate is not based on any measurement, moreover not only height of the pulse, but also its duration is relevant to relay contact longevity. And manufacturters normally factor in some derating anyhow, 30 need not be the exact longtime guaranteed value.....

In reply to # 3592451 by Lincspeed
Thanks to all for the relay tips. Found 3 old race car build relays (all 30 Amp) wired one up and damed if I didn't nearly break the windows in the garage!.

So what problems might I see not using a 40 Amp relay? I matched all wire sizes in the hook up.